Powder forged member, powder mixture for powder forging, method for producing powder forged member, and fracture split type connecting rod using the same

ABSTRACT

A member produced by powder forging which retains machinability and improved fatigue strength without having an increased hardness and can retain self conformability after fracture splitting; a powder mixture for powder forging; a process for producing a member by powder forging; and a fracture splitting connecting rod obtained from the member produced by powder forging. The member produced by powder forging is one obtained by preforming a powder mixture, subsequently sintering the preform, and forging the resultant sintered preform at a high temperature. The free-copper proportion in the sintered preform at the time when the forging is started is 10% or lower, and the member obtained through the forging has a composition containing, in terms of mass %, 0.2-0.4% C, 3-5% Cu, and up to 0.4% Mn (excluding 0), the remainder being iron and incidental impurities, and has a ferrite content of 40-90%.

TECHNICAL FIELD

The present invention relates to a powder forged member obtained bysubjecting a powder mixture to preliminary compacting, then sinteringthe subjected compacted preform, and thereafter forging the obtainedsintered preform, a powder mixture for powder forging, a method forproducing the powder forged member, and a fracture split type connectingrod produced using the powder forged member.

BACKGROUND ART

Conventionally, there has been widely carried out a powder forgingmethod for subjecting a powder mixture to preliminary compacting, thensintering the subjected compacted preform, and thereafter forging theobtained sintered preform to produce machine parts. Examples of typicalmachine parts produced by the powder forging method include a connectingrod and a bearing race. Typically, the component composition of thesemachine parts using a pure iron-based powder contains C: 0.45 to 0.65%by mass (hereinafter, “% by mass” is merely represented as “%”), and Cu:1.5 to 2% from the relationship of machinability and fatigue strength ofproducts on machining after forging, and the like. A method forincreasing the content of C or a method for increasing both the contentsof C and Cu is generally required for weight saving or increase offatigue strength of these machine parts. Although the fatigue strengthof the part is increased in the methods for increasing the content of C,the hardness is also increased. This causes a problem that the servicelife of a tool on machining after forging is remarkably reduced tounfortunately increase the product cost. In addition, there is adisadvantage that the increased content of Cu causes the generation ofcracks on forging easily.

A method for adding a reheating process and a cooling process after aforging process (see Patent Document 1), and a method for adding otheralloy elements such as Ni and Mo (see Patent Document 2) are disclosedas another method for increasing the fatigue strength of the machinepart. However, the former method causes the increase of processes andthe latter method uses expensive alloys, increasing the cost of the partand increasing the hardness of the part as in the method for increasingthe content of C. This causes a disadvantage that the machinability isreduced.

The above conventional methods decrease the toughness of the part withthe increase of the hardness, causing the fracture surface to tend tobecome flat. When the part is produced using a fracture dividing methodadopted in the connecting rod or the like, there is caused a particularproblem of easily generating the positional shift of the part onassembling the part (i.e., reducing self-consistency).

Patent Document 1: Japanese Unexamined Patent Publication No. 61-117203

Patent Document 2: Japanese Unexamined Patent Publication No. 60-169501

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is an object of the present invention to provide a powder forgedmember in which fatigue strength is improved while securing itsmachinability without increasing its hardness, and self-consistencyafter fracture split can be secured, a method for producing the same,and a fracture split type connecting rod using the powder forged member.

Means for Solving the Problems

In accordance with a first aspect of the present invention, a powderforged member has excellent machinability and fatigue strength, thepowder forged member obtained by forging a sintered preform at a hightemperature, the sintered preform formed by subjecting a powder mixtureto preliminary compacting and thereafter sintering the subjectedcompacted preform, the sintered preform having a ratio of free Cu of 10%or less upon the start of the forging, the component composition of thepowder forged member after the forging composed of, C: 0.2 to 0.4% bymass, Cu: 3 to 5% by mass and Mn: 0.5% by mass or less (excluding 0),and the balance iron with inevitable impurities, and the powder forgedmember having a ferrite ratio of 40 to 90%.

In the powder forged member, a relative density to theoretical densityis preferably 97% or more.

In the powder forged member, it is preferable that a hardness is HRC 33or less, and a partial pulsating tensile fatigue limit is 325 MPa ormore.

It is preferable that the powder forged member contains at least onemachinability-improving material in a total amount of 0.05 to 0.6% bymass, the machinability-improving material selected from the groupconsisting of MnS, MoS₂, B₂O₃ and BN.

In accordance with a second aspect of the present invention, a fracturesplit type connecting rod is produced by using the powder forged memberof the first aspect.

In accordance with a third aspect of the present invention, a powdermixture is used as a raw material for the powder forged member of thefirst aspect, wherein a component composition except a lubricant iscomposed of, C: 0.1 to 0.5% by mass, Cu: 3 to 5% by mass, Mn: 0.4% bymass or less (excluding 0), O: 0.3% by mass or less and the balance ironwith inevitable impurities.

It is preferable that the powder mixture for powder forging is obtainedby adding a graphite powder, a copper powder and a lubricant into aniron-based powder composed of, C: less than 0.05% by mass, O: 0.3% bymass or less and the balance iron with inevitable impurities.

In accordance with a fourth aspect of the present invention, a powdermixture is used as a raw material for the powder forged member of thefirst aspect, wherein a component composition except a lubricantcontains, C: 0.1 to 0.5% by mass, Cu: 3 to 5% by mass, Mn: 0.4% by massor less (excluding 0), O: 0.3% by mass or less, and also at least onemachinability-improving material in a total amount of 0.05 to 0.6% bymass, and the balance iron with inevitable impurities, themachinability-improving material selected from the group consisting ofMnS, MoS₂, B₂O₃ and BN.

It is preferable that the powder mixture for powder forging is obtainedby adding a graphite powder, a copper powder, at least onemachinability-improving material selected from the group consisting ofMnS, MoS₂, B₂O₃ and BN, and a lubricant into an iron-based powdercomposed of, C: less than 0.05% by mass, O: 0.3% by mass or less and thebalance iron with inevitable impurities.

In accordance with a fifth aspect of the present invention, a method forproducing the powder forged member having excellent machinability andfatigue strength of the first aspect, the method includes: a compactingand sintering step of subjecting the powder mixture for powder forgingof the third aspect to preliminary compacting and thereafter sinteringthe subjected compacted preform to form a sintered perform; and aforging step of forging the sintered preform at a high temperature toform a powder forged member.

In accordance with a sixth aspect of the present invention, a method forproducing the powder forged member having excellent machinability andfatigue strength of the first aspect includes: a compacting andsintering step of subjecting the powder mixture for powder forging ofthe fourth aspect to preliminary compacting and thereafter sintering thesubjected compacted preform to form a sintered perform; and a forgingstep of forging the sintered preform at a high temperature to form apowder forged member.

EFFECT OF THE INVENTION

The present invention increases the content of Cu as compared with thatof the conventional one instead of decreasing the content of C of thepowder forged member contrary to the conventional one, and limits theratio of free Cu in the sintered preform upon the start of the forging.Thereby, since soft ferrite is increased by the reduction of the contentof C to suppress the increase of hardness, the machinability can besecured and the toughness can be maintained to ensure self-consistencyafter fracture split. Furthermore, since the amount of diffusion of Cuinto ferrite is increased by the increase of the content of Cu and thelimit of the ratio of free Cu to promote solid solution strengthening,the fatigue strength is also drastically improved. The cracks of thepowder forged member on forging can be prevented by limiting the ratioof free Cu.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a perspective view showing the shape and size of a testpiece of a powder forged member used for fatigue test of Example, andFIG. 1( b) is a sectional view showing a section taken along line A-A.

FIG. 2 is a sectional view showing an applied state of a tensile load toa test piece of a powder forgedmember in fatigue test.

FIG. 3 is a graph showing the relationship between ratio of free Cu andfatigue limit.

FIG. 4 is a sectional view showing the microstructure of a powder forgedmember.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in further details.

[Composition of Powder Forged Member]

First, the reason of limiting the composition of a powder forged memberaccording to the present invention, that is, a component composition,structure, density and a ratio of free Cu in a sintered preform will bedescribed.

C: 0.2 to 0.4%

C is an indispensable element for ensuring the strength of a base steel.Conventionally, the hardness and strength of the base steel have beenincreased by increasing the content of C to decrease ferrite andincrease perlite in the structure of the base steel. On the contrary, inthe present invention, the content of C is conversely decreased to 0.4%or less in order to suppress the increase of the hardness of the basesteel. However, since the strength of the base steel cannot besufficiently ensured even if the content of Cu is increased when thecontent of C is excessively decreased, the content of C is set to 0.2%or more. Therefore, the content of C is set to 0.2 to 0.4%.

Cu: 3 to 5%

Cu is an element which is dissolved in a ferrite phase in the structureof a base steel on heating for sintering and forging to form a solidsolution to exhibit solid solution strengthening effect, and is partlyprecipitated on cooling to enhance the strength of the base steel. Inthe conventional product, Cu is almost used in an amount of about 2% ofsolid solution limit in the ferrite phase near the eutectoid temperatureof Fe—C system. On the other hand, the solid solution limit of Cu in anaustenite phase is about 8%. Cu of 3% or more can be dissolvedsufficiently in the base steel to form a solid solution by increasing aheating temperature as compared with that of the conventional productand/or extending heating time. In the present invention, a larger amountof Cu than that of the conventional product is dissolved in theaustenite phase to strengthen the solid solution of the ferrite phasegenerated in a cooling process. The content of Cu of less than 3.0%cannot exhibit the aimed solid solution strengthening effectsufficiently. On the other hand, the content of Cu exceeding 5.0% causesthe free Cu to remain easily. The extension of heating time such as theextension of sintering time is required to limit the ratio of free Cu to10% or less, and consequently the productivity is reduced. Therefore,the content of Cu is set to 3 to 5%, and preferably 3 to 4%.

Mn: 0.5% or Less (Excluding 0)

Mn is an element which has the deoxidizing effect of the base steel anduseful to increase hardenability and enhance the strength of the basesteel. However, Mn has a high affinity to oxygen, and reacts with oxygenin atmosphere in a powder producing process or in a sintering process ofa product subjected to preliminary compacting to easily produce anoxide. The content of Mn exceeding 0.5% makes it difficult to reduce aMn oxide and remarkably reduce the quality characteristics of the powderforged member such as the reduction of density and strength caused bythe Mn oxide. Therefore, the content of Mn is set to 0.5% or less(excluding 0), and preferably 0.4% or less (excluding 0).

Balance: Iron and Inevitable Impurities

The powder forged member of the present invention may contain P, S, Si,O, N and other elements as inevitable impurities.

Ratio of Free Cu: 10% or Less

As described above, Cu nearly two times that of the conventional productis used to strengthen the solid solution of the ferrite phase, andnon-dissolved Cu (i.e., free Cu) easily remains in the base steel.Therefore, forging cracks may be generated by hot brittleness onforging. In a severe case, the possibility of the damage of the sinteredpreform is increased on handling between a forming sintering process anda forging process. Therefore, in the present invention, the ratio offree Cu in the sintered preform upon the start of the forging is set to10% or less. Here, the ratio of free Cu, which means the ratio ofnon-dissolved Cu in the base steel, of the total amount of Cu added, canbe quantitated by the following method. That is, the section of thesintered preform as a member to be measured is ground by paper and abuff, and is then etched by picric acid. Three positions having a rangeof 0.2 mm×0.3 mm are photographed by 400 magnifications using an opticalmicroscope, and the total area of portions of copper color is measuredby image processing. On the other hand, the total area of portions ofcopper color of a reference material is measured by the same method. Asthe reference material, there is used a product obtained by sintering acompacted product compacted in the same component compositions, shapeand forming pressure as those of the member to be measured under thecondition of 1000° C. for 20 minutes where Cu is not dissolvedsubstantially in the base steel. The ratio of free Cu may be calculatedusing the following formula: Ratio of free Cu (%)=[total area ofportions of Cu color of member to be measured]/[total area of portionsof Cu color of reference material]×100.

Ferrite Ratio: 40 to 90%

When the powder forged member has a ferrite ratio of less than 40%, thepowder forged member has deficient toughness and insufficientself-consistency after fracture split. On the other hand, when thepowder forged member has a ferrite ratio exceeding 90%, the powderforged member has excessively high toughness and large elongation,causing deformation on fracture split to deteriorate dimensionalaccuracy. Therefore, the ferrite ratio of the powder forged member isset to 40 to 90%.

Relative Density to Theoretical Density: 97% or More

When the relative density to the theoretical density is less than 97%,the degree of reduction in the fatigue strength of the powder forgedmember becomes large. Therefore, the relative density to the theoreticaldensity of the powder forged member is preferably 97% or more. When therelative density is set to 97% or more, the hardness of the powderforged member becomes HRC 33 or less and the partial pulsating tensilefatigue limit becomes 325 MPa or more. Therefore, there is provided apowder forged member having secured machinability and excellent fatiguestrength.

Machinability-Improving Material: Total Amount of 0.05 to 0.6%

A machinability-improving material may be added on preliminarycompacting (i.e., to a powder mixture for powder forging) to improve themachinability of the powder forged member. As themachinability-improving material, for example, a powder composed of MnS,MoS₂, B₂O₃ or BN may be used. They may be used either singly or in theform of a combination of two or more members. When the amount of themachinability-improving material to be added is less than 0.05% in thetotal amount, the machinability-improving effect is not sufficientlyobtained. On the other hand, when the amount of themachinability-improving material to be added exceeds 0.6%, an areaoccupied by an iron material is reduced, and nonmetal as the startingpoint of fatigue cracks is increased, showing a tendency of reduction inthe fatigue strength. Therefore, the total amount of themachinability-improving material to be added is preferably 0.05 to 0.6%in the total amount.

[Component Composition of Powder Mixture for Powder Forging]

Next, the reason of limiting the component composition of the powdermixture for powder forging (hereinafter, merely referred to as a “powdermixture”) will be described.

C: 0.1 to 0.5%

It is necessary to adjust the content of C of the powder mixture inconsideration of the amount of oxygen in the powder mixture and the kindof atmosphere gas on sintering so that the content of C of the powderforged member finally obtained is set to 0.2 to 0.4%. That is, wheninactive gas atmosphere such as N₂ gas is used in the sintering process,C is oxidized and consumed by oxygen in the powder mixture andimpurities oxygen in atmosphere gas. The content of C of the sinteredpreform (i.e., the powder forged member) is lower than that of thepowder mixture. Thereby, the content of C of the powder mixture isadjusted to more than 0.2% and 0.5% or less which is higher than that ofthe powder forged member. On the other hand, when atmosphere gas havinghigh carbon potential such as endothermic gas is used, carburizationcaused by atmosphere gas usually advances to more than the amount ofoxidation consumption of C by oxygen in the powder mixture, and thecontent of C of the sintered preform (i.e., the powder forged member)becomes higher than that of the powder mixture. Thereby, the content ofC of the powder mixture is adjusted to 0.1% or more and less than 0.4%which is lower than that of the powder forged member. Therefore, thecontent of C of the powder mixture may be set in the range of 0.1 to0.5% while the change in the content of C is predicted in accordancewith the content of oxygen of the powder mixture and the kind ofsintering atmosphere gas.

O: 0.3% or Less

The variation of the consumed C amount is also larger when the contentof oxygen of the powder mixture is higher, and it becomes difficult toset the content of C of the powder forged member to the target of 0.2 to0.4%. Thereby, the content of oxygen of the powder mixture is set to0.3% or less.

Other Components

Cu, Mn and the machinability-improving material are not consumed andproduced on sintering as in C. The content of each of the components inthe powder mixture is defined as the same as the content of each of thecomponents in the powder forged member (although the value of thecontent of each of the components is extremely slightly changed by theincrease and decrease of the amount of C on sintering in a precisesense, the value is within an ignorable range).

[Method for Producing Powder Forged Member]

Next, a method for producing the powder forged member satisfying theabove composition will be described.

First, the change of the content of C on sintering is predicted inaccordance with the content of oxygen in an iron-based powder and thekind of sintering atmosphere gas. A graphite powder in which the contentof C of the powder mixture is in the range of 0.1 to 0.5% so that thecontent of C after sintering is set to 0.2 to 0.4%, a copper powder inwhich the content of Cu is 3 to 5%, and the machinability-improvingmaterial of the total amount of 0.05 to 0.6% if necessary are added intoan iron-based powder. A proper amount of a lubricant is further addedthereto to produce a powder mixture. This powder mixture is subjected topreliminary compacting by a pressure compacting machine to produce acompacted preform.

When the iron-based powder used in producing the powder mixture is lesscompressibility, the density of the compacted preform on preliminarycompacting is hardly increased. The inside of the sintered preform isoxidized during high temperature conveyance to the forging process aftersintering, and a phenomenon in which the strength of the sinteredpreform is reduced by an oxide film occurs even if the sintered preformis forged. Therefore, in order to soften the iron-based powder andincrease the density of the compacted preform to prevent the internaloxidation of the compacted preform, the content of C of the iron-basedpowder is set to be less than 0.05%, preferably 0.04% or less, and morepreferably 0.02% or less.

Then, this compacted preform is sintered at a high temperature toproduce a sintered preform. Here, referring to sintering condition,higher temperature and longer time are preferable because the diffusionof Cu advances and the amount of free Cu decreases as the temperature ishigher or as time is longer. However, when the content of Cu is, forexample, 4%, the ratio of free Cu can be set to 10% or less by sinteringthe preform at 1190° C. or more for 10 minutes.

This sintered preform is immediately forged with a predetermined forgingpressure at a high temperature without cooling the sintered preform toobtain a powder forged member. Higher forging pressure is preferablebecause the density of the powder forged member becomes higher and thestrength is increased as the forging pressure is higher. However, when aconnecting rod having a shape and size as shown in, for example, FIG. 1is formed, the relative density to the theoretical density can be set to97% or more by forging the preform with a pressure of 6.0 ton/cm² ormore, resulting in the powder forged member having excellentmachinability and fatigue strength.

Although the example immediately forging the preform using thetemperature after sintering is described in the producing method, thepreform may be once cooled after being sintered, and reheated to beforged. In this case, the preform is heated twice on sintering andforging and the heating time becomes longer inevitably. Thereby, evenwhen the heating temperature is a temperature (about 1050° C. to about1120° C.) further lower than the lower limit temperature (1190° C.), theratio of free Cu can be set to 10% or less.

A fracture split type connecting rod produced using this powder forgedmember has reduced tool abrasion on machining, and suppress the increasein cost of parts, and has excellent fatigue strength andself-consistency on assembling after fracture split.

Example 1 Influence of Ratio of Free Cu

A graphite powder and a copper powder were added into a pure iron-basedpowder having a component composition shown in Table 1 so that thecontents of C and Cu after being sintered were respectively 0.3% and 4%.Zinc stearate of 0.75% as a lubricant was further added thereto, andthey were mixed for 30 minutes to produce a powder mixture. The powdermixture was subjected to preliminary compacting with a compactingsurface pressure of 6 ton/cm² to produce a compacted preform.

TABLE 1 Components C Mn P S Si O N Content (mass %) 0.001 0.19 0.010.009 0.01 0.12 0.004

This compacted preform was dewaxed at 600° C. for 10 minutes under N₂gas atmosphere, and was then sintered at various temperatures of 1110 to1260° C. for 10 minutes to produce a plurality of sintered preforms. Theratio of free Cu of each of some sintered preforms was measured by usingthe method described in the above [Composition of Powder Forged Member]The remaining sintered preforms were immediately forged with a forgingpressure of 10 ton/cm² to produce test pieces of powder forged membersimitating the shape of a connecting rod. Burr of each of the test pieceswas removed, and the surface scale was removed by shot or the like toprovide the test pieces to a pulsating tensile fatigue test. FIG. 1shows the shape and size of each of the test pieces used for the fatiguetest. FIG. 2 shows an applied state of a tensile load to each of thetest pieces in the fatigue test.

Table 2 and FIG. 3 show measurement and test results. As is apparentfrom Table 2 and FIG. 3, as the sintering temperature is higher, theratio of free Cu decreases and the fatigue limit increases. When thesintering time is 10 minutes, the ratio of free Cu is 10% or less at thetemperature of 1190° C. or more, and the fatigue limit of 325 MPa ormore is obtained. FIG. 4 shows comparatively the cross-sectionalmicrostructures of a reference material having a ratio of free Cu of100%, a comparative material having the ratio of 15% and an inventivematerial of 3%. In FIG. 4, portions to which net hatching is appliedhave existing free Cu.

TABLE 2 Test Sintering Ratio of Fatigue pieces temperature free Cu limitNo. (° C.) (%) (MPa) Note 101 1110 82 245 Comparative 102 1140 56 275example 103 1170 43 294 104 1180 19 324 105 1190 9.8 353 Inventive 1061200 4.6 353 example 107 1230 2.1 363 108 1260 1.4 373

In Inventive Example, the ferrite ratio of the powder forged member wasabout 70% at any sintering temperature.

Example 2 Influence of Contents of C and Cu

A graphite powder and a copper powder were added into a pure iron-basedpowder having the same component composition as that of Example 1 shownin Table 1 with the addition amounts of the graphite powder and copperpowder variously changed so that the content of C and Cu after beingforged were respectively 0.1 to 0.6% and 2 to 5% to produce a powdermixture. The powder mixture was subjected to preliminary compacting inthe same condition as that of Example 1 described above to form acompacted preform. This compacted preform was dewaxed at 600° C. for 10minutes under N₂ gas atmosphere, and was then sintered at 1120° C. for30 minutes under N₂ gas atmosphere to produce sintered preforms. Thesintered preforms were heated at 1050° C. for 30 minutes under N₂ gasatmosphere, and was then forged with a forging pressure of 10 ton/cm² toproduce test pieces of powder forged members imitating the shape of thesame connecting rod as that of Example 1 described above. These testpieces were subjected to a tensile fatigue test in the same condition asthat of Example 1 described above, and the HRC hardness of each of thesurfaces of the test pieces after being machined was measured.

Furthermore, the following test was performed in order to quantifyself-consistency after fracture split. That is, a disk-shaped test pieceof a powder forged member having a diameter of 90 mm×a thickness of 40mm was produced in the same condition as in the above description. Thiswas machined to produce a ring-shaped test piece having an outerdiameter of 80 mm, an inner diameter of 40 mm×a thickness of 20 mm andhaving a V notch having a depth of 1 mm and an angle of 45 degrees on aninner ring diagonal line. This test piece was subjected to tensilefracture in the depth direction and right-angled direction of the notch.A real area including micro unevenness of the fracture surface wasmeasured by using an optical three-dimensional measurement device(produced by GFMesstechnik Company, type: MicroCAD 3×4), and a ratiorelative to a flat project area ignoring the unevenness (referred to asa “fracture split area ratio”) was calculated. Furthermore, the presenceor absence of the shift of the engaged position of the fracture surfaceafter fracture split was visually investigated.

Table 3 shows test results. The ratio of free Cu of each of the testpieces before being forged (upon the start of the forging) exceeded 10%in test piece No. 222 having the content of Cu exceeding 5%. However,the ratio was 10% or less in the other test pieces.

TABLE 3 Chemical Fracture- Test composition Fatigue Ferrite divisionpieces (mass %) Hardness limit ratio area ratio No. C Cu (HRC) (MPa) (%)(−) Self-consistency Note 201 0.10 2.0 11.7 200 97 1.54 X: deformationcaused Comparative 202 0.10 2.5 12.8 209 97 1.53 X: deformation causedExample 203 0.10 3.0 14.0 218 97 1.56 X: deformation caused 204 0.10 3.515.2 227 96 1.55 X: deformation caused 205 0.10 4.0 16.4 236 96 1.54 X:deformation caused 206 0.10 4.5 17.5 245 97 1.52 X: deformation caused207 0.10 5.0 18.7 255 98 1.51 X: deformation caused 208 0.20 2.0 16.2235 83.6 1.54 X: deformation caused 209 0.20 2.5 17.4 244 84.1 1.53 X:deformation caused 210 0.20 3.0 18.5 307 84.6 1.51 ◯ Inventive 211 0.203.5 19.7 316 85.1 1.50 ◯ Example 212 0.20 4.0 20.9 325 85.6 1.49 ◯ 2130.20 4.5 22.1 334 86.1 1.48 ◯ 214 0.20 5.0 23.2 341 86.6 1.46 ◯ 215 0.302.0 20.7 270 66.9 1.46 ◯ Comparative 216 0.30 2.5 21.9 280 67.4 1.45 ◯Example 217 0.30 3.0 23.1 340 67.9 1.47 ◯ Inventive 218 0.30 3.5 24.3346 68.4 1.45 ◯ Example 219 0.30 4.0 25.4 352 68.9 1.44 ◯ 220 0.30 4.526.6 357 69.4 1.43 ◯ 221 0.30 5.0 27.8 360 69.9 1.42 ◯ 222 0.30 6.0 28.0306 70.1 Not Not measured Comparative measured Example 223 0.40 2.0 25.3315 50.2 1.44 ◯ 224 0.40 2.5 26.4 360 50.7 1.43 ◯ 225 0.40 3.0 27.6 36351.2 1.42 ◯ Invention 226 0.40 3.5 28.8 365 51.7 1.41 ◯ Example 227 0.404.0 30.0 366 52.2 1.39 ◯ 228 0.40 4.5 31.1 367 52.7 1.38 ◯ 229 0.40 5.032.3 322 53.2 1.37 ◯ 230 0.50 2.0 29.8 343 33.5 1.40 ◯ Comparative 2310.50 2.5 32.5 347 34 1.37 ◯ Example 232 0.50 3.0 33.1 349 34.5 1.36 X:shift caused 233 0.50 3.5 33.3 358 35 1.36 X: shift caused 234 0.50 4.034.5 367 35.5 1.35 X: shift caused 235 0.50 4.5 35.7 376 36 1.34 X:shift caused 236 0.50 5.0 36.8 357 36.5 1.32 X: shift caused 237 0.602.0 34.3 366 16.8 1.35 X: shift caused 238 0.60 2.5 35.5 375 17.3 1.34X: shift caused 239 0.60 3.0 36.7 384 17.8 1.32 X: shift caused 240 0.603.5 37.8 394 18.3 1.31 X: shift caused 241 0.60 4.0 39.0 403 18.8 1.30X: shift caused 242 0.60 4.5 40.2 412 19.3 1.29 X: shift caused 243 0.605.0 41.4 200 19.8 1.28 X: shift caused

As shown in Table 3, the following is confirmed. Each of InventiveExamples in which the contents of C and Cu, the ferrite ratio and theratio of free Cu were within the range defined in the present invention,which had hardness of HRC 33 or less, had no problem in machinability.Each of Inventive Examples had fatigue limit of 300 MPa or more,specifically 325 MPa or more, except some of Inventive Examples (testpiece Nos. 210, 211). Inventive Examples had no shift observed in thefracture surface after fracture split and had no problem inself-consistency. Inventive Examples satisfied machinability, fatiguestrength and self-consistency after fracture split simultaneously.

On the other hand, in Comparative Examples in which the componentcomposition and/or the ferrite ratio fall/falls out of the range definedin the present invention, Comparative Examples, which have hardness ofHRC 33 or less, have fatigue limit up to 300 MPa except some ComparativeExamples (test piece Nos. 230,231) and cause deformation due toelongation in fracture split to reduce dimensional accuracy (test pieceNos. 201 to 209). On the other hand, in Comparative Examples havingfatigue limit of 300 MPa or more, the Comparative Examples have hardnessexceeding HRC33 and have deteriorated machinability, and cause engagedpositional shift of the fracture surface to cause a problem ofself-consistency. Therefore, it turns out that it is very difficult toobtain the powder forged member simultaneously satisfying machinability,fatigue strength and self-consistency after fracture split.

As shown in Table 3, the fracture split area ratio can be used as theindex representing self-consistency. When the fracture split area ratiois less than 1.37, the engaged shift of the fracture split surfaceoccurs easily. On the other hand, when the fracture split area ratioexceeds 1.51, it turns out that the deformation due to elongationbecomes remarkable and the dimensional accuracy is deteriorated.

Example 3 Influence of Relative Density

Next, there were produced test pieces of powder forged members havingthe same component composition (C: 0.3%, Cu: 3.5%) as that of test pieceNo. 218 of Example 2 in the same condition as that of Example 2 exceptthat only a forging pressure was variously changed in the range of 2.5to 10 ton/cm². The influence of the relative density of the powderforged member exerted on the fatigue limit was investigated. While thefatigue limit was measured, the HRB hardness of each of the test pieceswas also measured. Table 4 shows test results.

TABLE 4 Test Forging Relative Fatigue pieces pressure density Hardnesslimit No. (ton/cm²) (%) (HRB) (MPa) 218 10 99 105.0 346 301 7.5 98 100.0338 302 9.5 99 101.5 340 303 6.0 97 97.0 329 304 4.0 95 91.5 316 305 3.594 86.5 299 306 2.5 93 80.0 286

As shown in above Table 4, it is confirmed that the fatigue limit of 325MPa or more could be ensured when the relative density to thetheoretical density was 97% or more.

Example 4 Influence of Machinability-Improving Material

Next, test pieces of powder forged members having the same componentcomposition (C: 0.3%, Cu: 3.5%) as that of the test piece No. 218 ofExample 2 as in Example 3 were produced in the same manner as in Example2 except that various machinability-improving materials were added withthe addition amount thereof changed. The influence exerted onmachinability was investigated. Referring to machinability, a thrustforce was measured when a hole was formed from the surface of the testpiece at the number of rotations of 200 rpm and the cutting speed of0.12 mm/rev using an SKH drill having a diameter of 5 mm. This was usedas the index of machinability. Table 5 shows the measurement results.

As is apparent from Table 5, the thrust force is reduced with theincrease of the addition amount of the machinability-improving materialto improve the machinability. However, when the addition amount of themachinability-improving material exceeds 0.6%, the large decrease trendof the fatigue limit is observed even in any machinability-improvingagent.

TABLE 5 Machinability - improving material Test Amount to be ThrustFatigue pieces added force Hardness limit No. Kinds (mass %) (N) (HRC)(MPa) 218 — 0.0 770 24.3 346 401 MnS 0.2 765 24.8 351 402 0.4 755 25.2350 403 0.6 750 26.2 335 404 0.8 750 26.5 306 405 MoS₂ 0.8 750 25.5 308406 0.6 750 25.8 338 407 B₂O₃ 0.6 739 24.3 334 408 0.8 744 25.4 299 409BN 0.6 746 24.9 336 410 0.8 749 26.3 316

Example 5 Influence of Oxygen Content of Powder Mixture

Next, the content of oxygen of a powder mixture was changed using aniron-based powder having different content of oxygen, and test pieces ofpowder forged members were produced in the same condition as in that ofEmbodiment 1 described above. The contents of C and Cu of the powdermixture after being forged were respectively set to 0.3% and 4% as thetarget, and the addition amount of graphite powder was set to0.3%+(content % of oxygen of iron-based powder−0.05%)×¾ to adjust thecontent of C. Referring to this test piece, the content of C and thefatigue limit were measured, and the influence of the content of oxygenof the powder mixture exerted thereon was investigated.

Table 6 shows test results. As shown in Table 6, when the content ofoxygen of the iron-based powder (i.e., the powder mixture) was 0.3% orless (test piece Nos. 501 to 503), the content of C of the powder forgedmember was an approximate target content of C. However, when the contentof oxygen of the iron-based powder (i.e., the powder mixture) exceeded0.3% (test piece No. 504), it turned out that the content of C of thepowder forged member was significantly shifted from the target contentof C and fell out of the appropriate range (0.2 to 0.4%) of the contentof C defined in the present invention to drastically reduce the fatiguestrength.

TABLE 6 Powder forged member Component Test Chemical composition ofcomposition Fatigue pieces iron-based powder (mass %) (mass %) limit No.C Mn P S Si O C Cu (MPa) Note 501 0.001 0.19 0.01 0.009 0.01 0.012 0.314.00 352 Inventive 502 0.001 0.18 0.01 0.009 0.01 0.020 0.29 4.05 353Example 503 0.001 0.18 0.01 0.009 0.01 0.030 0.30 4.00 351 504 0.0010.19 0.01 0.009 0.01 0.040 0.15 3.95 267 Comparative Example

Example 6 Influence of Content of C of Iron-Based Powder

Next, an iron-based powder having different content of C was used, and apowder mixture having the same component composition was produced byadjusting the addition amount of a graphite powder. Compacted preformsand test pieces of powder forged members were produced in the samecondition as in Embodiment 1 described above. The contents of C and Cuafter being forged were respectively set to 0.3% and 4% as the target.The densities of the compacted preform and powder forged member, and thefatigue limit of the powder forged member were measured.

Table 7 shows test results. As is apparent from Table 7, the decreasetrend of the density of the compacted preform is shown with the increaseof the content of C of the iron-based powder. When the content of C ofthe iron-based powder is 0.05% (test piece No. 604), it turns out thatthe fatigue strength is drastically reduced although the density of thepowder forged member after being forged is almost the same as that of acase where the content of C is less than 0.05% (test piece No. 601 to603).

TABLE 7 Powder forged Compacted member Test Component composition ofperform Fatigue pieces iron-based powder (mass %) Density Density limitNo. C Mn P S Si O (g/cm³) (g/cm³) (MPa) Note 601 0.001 0.19 0.01 0.0090.01 0.12 7.05 7.83 353 Inventive 602 0.005 0.18 0.01 0.008 0.01 0.126.90 7.83 352 Example 603 0.02 0.19 0.01 0.009 0.01 0.13 6.60 7.81 335604 0.05 0.20 0.01 0.009 0.01 0.12 6.30 7.79 279 Comparative Example

1. A powder forged member having excellent machinability and fatiguestrength, the powder forged member obtained by forging a sinteredpreform at a high temperature, the sintered preform formed by subjectinga powder mixture to preliminary compacting and thereafter sintering thesubjected compacted preform, the sintered preform having a ratio of freeCu of 10% or less upon the start of the forging, the componentcomposition of the powder forged member after the forging composed of,C: 0.2 to 0.4% by mass, Cu: 3 to 5% by mass, Mn: 0.5% by mass or less(excluding 0), and the balance iron with inevitable impurities, and thepowder forged member having a ferrite ratio of 40 to 90%.
 2. The powderforged member having excellent machinability and fatigue strengthaccording to claim 1, wherein a relative density to theoretical densityis 97% or more.
 3. The powder forged member having excellentmachinability and fatigue strength according to claim 2, wherein ahardness is HRC 33 or less, and a partial pulsating tensile fatiguelimit is 325 MPa or more.
 4. The powder forged member having excellentmachinability and fatigue strength according to any one of claims 1 to3, wherein the powder forged member contains at least onemachinability-improving material in a total amount of 0.05 to 0.6% bymass, the machinability-improving material selected from the groupconsisting of MnS, MoS₂, B₂O₃ and BN.
 5. A fracture split typeconnecting rod produced by using the powder forged member according toclaim
 1. 6. A powder mixture used as a raw material for a powder forgedmember according to any one of claims 1 to 3, wherein a componentcomposition except a lubricant is composed of, C: 0.1 to 0.5% by mass,Cu: 3 to 5% by mass, Mn: 0.4% by mass or less (excluding 0), O: 0.3% bymass or less and the balance iron with inevitable impurities.
 7. Thepowder mixture for powder forging according to claim 6, wherein thepowder mixture is obtained by adding a graphite powder, a copper powderand a lubricant into an iron-based powder composed of, C: less than0.05% by mass, O: 0.3% by mass or less and the balance iron withinevitable impurities.
 8. A powder mixture for powder forging used as araw material for a powder forged member according to claim 4, wherein acomponent composition except a lubricant contains, C: 0.1 to 0.5% bymass, Cu: 3 to 5% by mass, Mn: 0.4% by mass or less (excluding 0), O:0.3% by mass or less, and at least one machinability-improving materialin a total amount of 0.05 to 0.6% by mass, and the balance iron withinevitable impurities, the machinability-improving material selectedfrom the group consisting of MnS, MoS₂ and B₂O₃ and BN.
 9. The powdermixture for powder forging according to claim 8, wherein the powdermixture is obtained by adding a graphite powder, a copper powder, atleast one machinability-improving material selected from the groupconsisting of MnS, MoS₂, B₂O₃ and BN, and a lubricant into an iron-basedpowder composed of, C: less than 0.05%% by mass or less, O: 0.3%% bymass or less or less and the balance iron with inevitable impurities.10. A method for producing a powder forged member having excellentmachinability and fatigue strength, the method comprising: a compactingand sintering step of subjecting a powder mixture for powder forgingaccording to claim 6 to preliminary compacting and thereafter sinteringthe subjected compacted preform to form a sintered perform; and aforging step of forging the sintered preform at a high temperature toform a powder forged member.
 11. A method for producing a powder forgedmember having excellent machinability and fatigue strength, the methodcomprising: a compacting and sintering step of subjecting a powdermixture for powder forging according to claim 7 to preliminarycompacting and thereafter sintering the subjected compacted preform toform a sintered perform; and a forging step of forging the sinteredpreform at a high temperature to form a powder forged member, whereinthe powder forged member contains at least one machinability-improvingmaterial in a total amount of 0.05 to 0.6% by mass, themachinability-improving material selected from the group consisting ofMnS, MoS₂, B₂O₃ and BN.
 12. A method for producing a powder forgedmember having excellent machinability and fatigue strength, the methodcomprising: a compacting and sintering step of subjecting a powdermixture for powder forging according to claim 7 to preliminarycompacting and thereafter sintering the subjected compacted preform toform a sintered perform; and a forging step of forging the sinteredpreform at a high temperature to form a powder forged member.
 13. Amethod for producing a powder forged member having excellentmachinability and fatigue strength, the method comprising: a compactingand sintering step of subjecting a powder mixture for powder forgingaccording to claim 8 to preliminary compacting and thereafter sinteringthe subjected compacted preform to form a sintered perform; and aforging step of forging the sintered preform at a high temperature toform a powder forged member.